Handling of config structs with tagged struct fields

[hariso]

Ecdysis makes it possible to build flags from a struct, and in #1 we added support for parsing nested structs. We found that the following case is possible and needs to be properly handled:

type Config struct {
  DBConfig DBConfig `long:"database config"` 
}
type DBConfig struct {
  URL string `long:"db url"`
}

Nested structs are not flags on their own, they are merely a way to group flags. The goal of this issue is to clarify the usage of tags on nested structs (we can either forbid them, or allow them if we have a way to describe groups of flags).